The igneous rocks of Singapore: New insights to Palaeozoic and Mesozoic assembly of the Sukhothai Arc

Six plutons of granitic to gabbroic rocks have been recognised in the poorly exposed ground of north and east Singapore using new data from borehole cores and field observations. Five of the plutons were emplaced sequentially during the period 285-230 Ma, and these have been grouped within a single parent unit (Bukit Timah Centre) in a new lithodemic framework for Singapore. These plutons record the development of early Permian to Triassic arc-related magmatism towards the southern end of the Sukhothai Arc system of Thailand, Peninsular Malaysia and Indonesia. Compositional trends in the plutons record decreasing crustal contribution as the arc matured. The volcanosedimentary succession underlying southwest Singapore records contemporaneous deposition in the forearc basin, and includes pyroclastic units of sufficient thickness and extent to be assigned 'member' status in a new lithostratigraphical framework for Singapore. The largest pyroclastic unit, which is > 150 m thick, developed as volcanic activity peaked at c. 242 Ma and is correlated with a large (probably caldera-forming) eruption of one of the Permo-Triassic plutons. The composition, typology and age of the Bukit Timah Centre plutons confirm their affinity with Eastern Belt intrusions of the Eastern Province (one of three granitoid provinces in Southeast Asia), and support a widely accepted model that Eastern Province granitoids formed in an Andean-type setting as Palaeo-Tethys crust descended beneath the Indochina-East Malaya block. Widespread development of hydrothermal-tuffisite in the Permo-Triassic intrusions is correlated tentatively with rapid uplift following slab breakoff, when arc activity ceased in the Singapore region in the interval c. 230-205 Ma. A sixth pluton, much younger than (and unrelated to) the Bukit Timah Centre, was emplaced in northeast Singapore in the Upper Cretaceous.